Hitherto, JIS SUS630, which is a precipitation hardening type martensitic stainless steel, has been used as a steel for the turbine blades of steam turbines for use in thermal electric power plants.
The longer the final-stage turbine blades (moving blades) in low-pressure turbines or the like, the more the blades are effective from the standpoint of the energy efficiency of the steam turbines.
In recent years, there is a growing intense desire for improvements in the energy efficiency of steam turbines used in thermal electric power plants, and it is becoming increasingly necessary, with this trend, to further increase the length of turbine blades, i.e., further elongate turbine blades.
Incidentally, a further elongation of turbine blades results in an increase in the centrifugal force imposed on the turbine blades.
Turbine blades are hence required to not only have high strength sufficient to enable the turbine blades to withstand the increased high centrifugal force but also have impact resistance, i.e., resistance to collisions of foreign matter, e.g., separated scales.
Specifically, from the standpoint of coping with elongations of turbine blades, in particular, a further elongation of final-stage turbine blades, it is desirable that a steel for turbine blades should have a strength as high as 1,450 MPa or above in terms of 0.2% proof stress and a toughness as high as 15 J or above in terms of Charpy impact value (absorbed energy).
In this regard, SUS630, which has conventionally been used as a steel for turbine blades, is insufficient in strength although sufficient in toughness. There has hence been a desire for development of a material which has even higher strength while retaining the high toughness of SUS630.
As a prior-art technique which is relevant to the present invention, the following patent document 1 discloses a titanium-based alloy that contains, in terms of % by weight, 4-8% of aluminum, 4-8% of vanadium, and 1-4% of tin, as a material for accommodating elongations of turbine blades.
However, this material has a 0.2% proof stress as poor as 94.5 kg/mm2 or less, and is still insufficient in strength.
In addition, this alloy is a titanium-based alloy and is different from the steel of the invention, which will be described later.
Meanwhile, the following patent document 2 discloses, as a material for the final-stage moving blades of low-pressure turbines, a martensitic steel which contains, in terms of % by weight, 0.19-0.25% of carbon, up to 0.1% of silicon, up to 0.4% of manganese, 8.0% or more and less than 13.0% of chromium, more than 2% and 3.5% or less of nickel, more than 2% and 3.5% or less of molybdenum, 0.05-0.35% of vanadium, 0.02-0.20% of one or two of niobium and tantalum, and 0.04-0.15% of nitrogen and which has a wholly tempered martensite structure.
However, this material has too high hardness after a solution treatment because of the high carbon content and hence has poor productivity. In addition, there is a possibility that the chromium in the matrix is consumed by carbon during the formation of carbides, resulting in a decrease in corrosion resistance.
Moreover, this material differs from the steel of the present invention in the ranges of carbon and nickel contents, and is different from the present invention.
Furthermore, the following patent document 3 discloses, as a material for accommodating turbine blade elongations, a steel which contains, in terms of % by weight, 0.19-0.32% of carbon, up to 0.5% of silicon, up to 1.5% of manganese, 8-13% of chromium, 2-3.5% of nickel, 1.5-4% of molybdenum, 0.05-0.35% of vanadium, 0.02-0.3% of one or two of niobium and tantalum, and 0.04-0.15% of nitrogen and in which the value of Mo/C is 5-22.
This steel disclosed in patent document 3 also has a high carbon content and has the same problems as the steel disclosed in patent document 2. Moreover, this steel differs from the present invention in the contents of carbon and nickel.
As still another conventional technique relevant to the present invention, the following patent document 4 discloses a high-strength corrosion-resistant steel characterized by comprising, in terms of % by weight, up to 0.15% of carbon, up to 1% of silicon, up to 2% of manganese, 9-15% of chromium, 6-11% of nickel, 1-4% of molybdenum, 0.1-5% of copper, 0.5-2% of aluminum, and 0.001-0.1% of nitrogen, with the remainder being iron and incidental impurities.
However, this steel differs from the present invention in that this steel is intended to be used in applications such as fasteners for aircraft, parts for petrochemical apparatus, etc., and that this steel has a copper content as high as 0.1-5% and does not satisfy all of the expression (1), expression (2), and expression (3) according to the present invention which will be described later.
The following patent document 5 discloses a martensitic stainless steel excellent in terms of strength, spring properties, and formability, the stainless steel being characterized by containing, in terms of % by weight, 10-19% of chromium, 5.5-10% of nickel, up to 0.4% of silicon, up to 2.0% of manganese, 1.10-2.00% of aluminum, 0.5-2.0% of titanium, up to 0.03% of carbon, and up to 0.04% of nitrogen and satisfying Cr+2Ni+Mn+Al≦35%, 2Ni+Mn≧11%, and Cr+Al≧11.10%, with the remainder being iron and incidental impurities.
The steel disclosed in patent document 5 also differs from the present invention in that this steel is intended to be used in applications such as gasket materials for engines or chemical plants, etc., that this steel contains titanium as an alloying element in an amount as large as 0.5-2.0%, and that this steel does not satisfy all of the expression (1), expression (2), and expression (3) according to the present invention.
Moreover, the following patent document 6 discloses a martensitic stainless steel characterized by having a composition which contains, in terms of wt %, up to 0.07% of carbon, up to 1.5% of silicon, 0.2-5% of manganese, 0.01-0.4% of sulfur, 10-15% of chromium, 7-14% of nickel, 1-6% of molybdenum, 1-3% of copper, 0.3-2.5% of titanium, 0.2-1.5% of aluminum, and up to 0.1% of nitrogen, with the remainder being iron and impurities commonly present, and by containing titanium sulfide.
The steel disclosed in patent document 6 also differs from the present invention in that this steel is intended to be used in applications such as springs and the like, that the steel contains copper and titanium as alloying elements in amounts as large as 1-3% and 0.3-2.5%, respectively, and that this steel does not satisfy all of the expression (1), expression (2), and expression (3) according to the present invention.
Furthermore, the following patent document 7 discloses a martensitic stainless steel characterized by having a composition which contains, in terms of % by weight, 9%≦Cr≦13%, 1.5%≦Mo≦3%, 8%≦Ni≦14%, 1%≦Al≦2%, 0.5%≦Ti≦1.5% with the proviso that Al+Ti≧2.25%, (detection limit)≦Co≦2%, (detection limit)≦W≦1% with the proviso that Mo+(W/2)≦3%, (detection limit)≦P≦0.02%, (detection limit)≦S≦0.0050%, (detection limit)≦N≦0.0060%, (detection limit)≦C≦0.025%, (detection limit)≦Cu≦0.5%, (detection limit)≦Mn≦3%, (detection limit)≦Si≦0.25%, and (detection limit)≦O≦0.0050%, and by satisfying Ms (° C.)=1302−42Cr−63Ni−30Mo+20Al−15W−33Mn−28Si−30Cu−13Co+10Ti≧50 and further satisfying (Cr equivalent)/(Ni equivalent)≦1.05 with the proviso that Cr equivalent (%)=Cr+2Si+Mo+1.5Ti+5.5Al+0.6W and Ni equivalent (%)=2Ni+0.5Mn+30C+25N+Co+0.3Cu.
The steel disclosed in patent document 7 also differs from the present invention in that this steel contains titanium as an alloying element in an amount as large as 0.5-1.5% and that this steel does not satisfy all of the expression (1), expression (2), and expression (3) according to the present invention.
Patent Document 1: Japanese Patent No. 3666315
Patent Document 2: Japanese Patent No. 3661456
Patent Document 3: Japanese Patent No. 3793667
Patent Document 4: JP-A-59-222558
Patent Document 5: JP-A-2-310339
Patent Document 6: JP-T-2008-525637 (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.)
Patent Document 7: JP-T-2008-546912